An indoor illumination lamp disclosed in JP 5162420 B has been proposed as a conventional indoor illumination lamp to be installed on a ceiling in an interior of a vehicle or the like. As illustrated in FIGS. 10 to 12, the conventional indoor illumination lamp 100 includes a housing 101 including a ceiling attachment unit 102 which can be attached to a ceiling in the interior of a vehicle, a lens cover 110 attached below the housing 101, and a function unit 120 mounted on the upper side of the housing 101.
The housing 101 is provided with a light source opening 103, and an attachment wall 104 erected to surround the light source opening 103. The function unit 120 is mounted in the attachment wall 104. At the function unit 120, a light source 121 is provided at a position facing the light source opening 103. Light from the light source 121 travels through the light source opening 103 and is transmitted through the lens cover 110, and the vehicle interior is irradiated with the light.